The present invention relates to inspection apparatus for optically inspecting the surface of a workpiece, and also to a surface illumination system which is particularly useful in such inspection apparatus. The invention is especially useful in the high speed, automated, optical inspection of printed circuit boards, wafers and the like, for detecting faults which require reworking, and therefore the invention is described below with respect to this application; it will be appreciated, however, that the invention, or features thereof, could advantageously be used in other applications as well.
The conventional inspection apparatus for optically inspecting the surface of a workpiece, such as a printed circuit board or wafer, in order to detect faults, typically includes a memory for storing reference data relating to the desired features of the workpiece surface, an illumination system for illuminating the workpiece surface, an optical sensor for sensing the light reflected from the illuminated workpiece surface and for outputting electric signals corresponding thereto, and a processor including logic circuitry for analyzing the electric signals outputted by the optical sensor for comparing them with the data stored in the memory, and for providing an indication of any discrepancies with respect thereto indicating a defect in the inspected workpiece surface. The reference data, concerning the desired features of the workpiece surface to be compared with the sensed workpiece surface, may relate to stored images of the desired workpiece surface, or to stored design rules for the design of such workpiece surface.
Since the workpiece surfaces are not perfectly flat, but rather exhibit some degree of surface relief such as grooves, scratches, and angled surfaces, it would be desirable that the illumination system include a Lambertian diffuser, namely a perfect diffuser effective to cause the intensity of reflected radiation to be indepedent of direction. Such illumination would produce spatial uniformity of the light (i.e. a uniform "sky of illumination") above the workpiece and thereby eliminate shadows caused by the relief in the workpiece surface. However, Lambertian diffusers are extremely wasteful of light, and therefore such a diffusing surface would require an extremely intense light source and extremely high power, or would substantially slow the operation of the inspection apparatus in order to obtain workable signals having the required signal-to-noise ratio.
The existing optical-inspection systems therefore provide focussed "Quasi-Lambertian" illumination. Examples of such illumination systems are described in our prior Israel Patent Application 81459, filed Feb. 2, 1987, and the later Chadwick et al U.S. Pat. No. 4,877,326 filed Feb. 19, 1988. The foregoing systems are designed to provide a "full sky" of illumination over essentially a linear field-of-view; i.e., one dimension of the field is much longer than the other.
In such linearly symmetric illumination systems, the illumination is substantially uniform along the longitudinal axis of the line of sensor elements, and also along the line perpendicular to the longitudinal axis. However, the uniformity drops significantly between these two lines, particularly along lines at 45.degree. from these two lines. Thus, while the linearly symmetric illumination in the existing inspection systems is quite satisfactory for systems with resolution elements, e.g., pixels, of 5-6 microns, they are less than satisfactory with respect to higher resolution inspection systems, e.g., having pixels of about 0.5 microns.